Perhaps a partial explanation for this tele lust is that many newcomers to the world of DSLRs are used to the 10x or more zooms of some compact cameras. Suddenly, an 18-55 (3x zoom) lens is not good enough.
Up to focal lengths of about 300mm, one can find reasonably many and cheap options. The 55-200 that I mentioned is one - later came the 55-300, and of course there are the 70-300 models. For those wanting to go longer, things get complicated. There are some lenses like some old Tamron and Sigma primes who will give you the extra reach, but when we move into the >300mm territory, you should ask yourself if you really need all that reach.
|A very cheap generic TC|
|And autofocus 2X Vivitar TC|
Teleconverters are an option, and for many beginners it feels like a huge deal. Suddenly, your 200mm lens becomes a 400mm lens for only a few dollars - used teleconverters online can be found with as little as 10$. Some even go a step further and try to attach a 2x teleconverter to those 400mm Sigmas. But of course there is no such thing as a free lunch...
2x teleconverters double the focal length, but reduce the light which passes through the lens. As such, a 400mm f/5.6 lens suddenly becomes an 800mm f/11. Let's make some calculations now, shall we? 800mm on a DX camera means 1200mm. Which means, if you plan to use this lens hand-held, you need a shutter speed of at least 1/1200 - so, 1/1250. With f/11, that means that you need ISO 1600 to take photos in direct sunlight! You can try to imagine what means for, say, overcast.
Not to mention, a wide-open tele paired with a bad quality teleconverter, will give you a very soft, substandard image. So, if the above scenario seemed pretty bad, imagine having to stop down even further. In other words, forget teleconverters without a tripod - and a good one, actually.
|Photo taken with a Sigma 400mm + 2x TC. On a sturdy tripod and closed down couple of stops, image quality is acceptable, barely|
Of course there are good teleconverters as well. Usually the modern, expensive ones made by Nikon, which also provide support for autofocus, EXIF data, and all the goodies modern lenses have. But still, let's not forget that no matter how good a teleconverter is, it will degrade the quality of a lens. There's no going around this. If you really, really need the reach, consider a cheap(er) telezoom rather than an expensive lens with an expensive teleconverter. Sigma makes some OK telezooms that might be what you need (if you're an amateur, they should be fine; if you're a professional, making money out of assignments/photos for which you need the reach, then you invest in a good but pricey lens. As simple as that)
|This one was taken with a 2x TC paired with a Nikkor 200mm f/4...|
|...just like this one|
So, to conclude, I don't favor teleconverters. Truth be told, I don't need any telelenses for my work. But I have tried a lot of teleconverters and a lot of telelenses, and I know that you have to spend quite a lot of money for combos like these to work sufficiently well.